The present invention relates to a method of producing a temperature sensor, in particular to a method in which lead wires of a non-noble metal and/or of alloys comprising a non-noble metal are used, the lead wires having at least partially been chemically gilded or gold-coated, e.g. by means of a currentless gold-plating method.
Temperature sensors in thin-film technology, such as platinum temperature sensors, have been produced in different designs and used for precise temperature measuring tasks for many years now.
A known temperature sensor is shown in FIG. 3, with FIG. 3(a) showing a cross-sectional illustration of the temperature sensor and FIG. 3(b) showing a top view illustration of the temperature sensor. A platinum film 102 of about 1 μm thickness is applied on an AL2O3 ceramic carrier 100. This platinum film 102 is structured such that it comprises a resistive trace of e.g. 100Ω. For protection of the platinum film 102, it is coated with a suitable protective layer 104. At two contact pads 106 shown in FIG. 3(b), lead wires 108 are welded on. So as to ensure sufficient mechanical stressability of the lead wires 108, as it is necessitated in the further processing of the sensor or in its employment, for example, a fixing glaze 110 by which the lead wires 108 undergo additional mechanical fixing is applied. The fixing glaze 110 is fired at temperatures of about 800° C. (the glaze has to melt) to ensure an operating temperature of up to 600° C., since the glaze is not allowed to soften during operation. For even higher operating temperatures, e.g. 800° C., correspondingly higher-melting glazes are employed.
Apart from the employment of platinum resistive traces, other metals may also be employed for the resistive trace. In the previously known designs of the temperature sensors described on the basis of FIG. 3, particularly due to the high firing temperature, either pure noble metal wires or wires of a noble metal alloy were used as lead wires, since these do not oxidize even at the higher process temperatures. Here, particularly platinum, palladium or silver are considered with respect to noble metal materials, and a gold-palladium alloy was used as noble metal alloy. Alternatively, also wires with sufficiently thick noble metal coating are used, whereby oxidation of the non-noble metal core may also be prevented at higher process temperatures, wherein a nickel wire having a platinum sheathing is used here, for example.
In further designs, as are described in DE 100 20 931 C1, for example, pure nickel wires without platinum sheathing, which have turned out to be specially inexpensive, are used as lead wires. However, a particular problem is to be taken into account here, arising due to the use of non-noble metals during the production of the temperature sensors. For example, if nickel wires are employed, the effect of the nickel chemically reacting with the fixing glaze mass of the fixing droplet occurs in the firing process of the fixing glaze droplet for wire fixing, when the melting phase is reached during the firing process. Due to this chemical reaction, many small bubbles in the wetting area to the wire surface develop in the glaze mass surrounding the lead wires at the contact pads of the sensor chip. These bubbles reduce the fixing quality, i.e. the maximum allowable tensile forces at the lead wires are decreased significantly thereby.
So as to take this problem into account, the fixing glaze is not applied immediately after attaching, e.g. by welding on, the lead wires in this method, but the applied nickel wires at first are oxidized in a further step after the attaching, in contrast to the above-described known sensors. Only then are the nickel wires, now provided with an oxide layer, provided with the fixing glaze at the contact pad in the usual manner, and is the same fused in a further firing process. The oxide layer, which has formed on the entire length of the nickel wire, then is removed in a further process step in the exposed wire area, i.e. the area not covered by the fixing mass, by way of a reduction process, e.g. in a N2/H2 atmosphere at about 600° C., since the oxide layer would be disturbing at this point for a further connection of the sensor to a measurement cable or another suitable connection site.
By way of the procedure just described, good fixing of even non-noble metal wires to a temperature measurement sensor or temperature sensor of the above-described type can be achieved.
Although it has been shown that the exposed wire ends can be tinned very well immediately after the reduction in the N2/H2 atmosphere, this complete or partial tinning of the exposed wire portions is not advantageous for all types of further processing. In fact, this tinning is advantageous for unproblematic further processing when soldering the sensors to a measurement cable or to another contact pad, but limits the operating temperature of the sensor to about 200° C., depending on the solder used. Furthermore, tinning also has to take place immediately after the reduction, since later oxidation of the lead wires would prevent tinning.
For applications at higher temperatures, a soft solder connection is not suited. In such cases of application, hard solder connections or weld connections are used for lead extension of the sensors, so that pre-tinning does not make sense in such a context. Although it would be the simplest solution to do without the tinning, it has been found, in connection with the reduction process, that it indeed leads to the oxygen being removed from the oxidized nickel surfaces, but accompanied by somewhat roughening the wire surface, which hereby again is very sensitive to renewed oxidation. In fact, such renewed oxidation also occurs even at moderate temperatures, e.g. during prolonged storage at room temperature, and particularly in the case of elevated humidity. In FIG. 3, the oxide layers on the lead wires 108 developing in the case of prolonged storage are designated with the reference numeral 112.
It is problematic in the above-described conventional sensors that different process steps are necessitated after completing the actual sensor, depending on the further use (soft soldering method or hard soldering method, welding). An additional process step may be needed also in the further processing, e.g. prior to the actual connecting, wherein the sensor is prepared for the desired contacting.
DE 91 08 274 U1 describes an infrared detector having a cylindrical housing consisting of a housing pot and a housing socket. A carrier plate of ceramics is held inside the housing by several plateau support pins, which are guided axially outward from the interior of the housing through the housing socket. These plateau support pins are formed as gilded contacting pins.
DE 41 07 142 A1 describes a method of producing a noble metal coating on a thin metal layer consisting of at least 70 wt.-% iron, cobalt and/or nickel and the crystallites thereof having a grain size of a maximum of 0.5 μm, wherein this metal layer is coated with a noble metal layer of 0.1 to 10 μm thickness in external-current-less or galvanic manner, wherein the non-noble metal layer is heated in the temperature range from 500° C. to 1300° C. in a non-oxidizing atmosphere prior to coating with the noble metal, until at least 30 area-% of the metal have a grain size of at least 2 μm or at least 50 area-% have a grain size of at least 1 μm. After cooling the non-noble metal layer, it is coated with the noble metal coating.